Caption: Sir William Crookes (1832-1919), British physicist and chemist. Crookes used vacuum tubes to investigate cathode rays, inventing his own vacuum tube, the Crookes tube. He discovered that the rays (which were made of electrons) made the sides of the glass tube fluoresce. He also showed that they travelled in straight lines and could be deflected by a magnetic field. Crookes invented the spinthariscope, which measured alpha radiation using scintillation (light flashes) caused by these charged particles. Crookes also discovered the element thallium.